Flow control valves

hello i am a mech engg student, i have few doubts in fluid dynamics and thermal engg subjects.. i would like to be cleared of them by this discussion forums
could anyone please explain the principle of flow control using valves.....waiting for .....

Understanding "Cv" is important to sizing control valves. This is GPM of water that a valve will pass for a pressure drop of 1 psi. Once you know this, you can determine using the Cv calculations how much flow that same valve will pass at other pressure drops, or determine what pressure drop you can expect from other flows.Cv Calculations